Thermionic valve circuit



P 5, 1939- w. s. PERCIVAL 2,171,671

THERMIONIC VALVE CIRCUIT Filed May 17, 1937 2 SheetsSheet l T g .L

FILTER HIGH PASS FILTER & 4 a? O---'WVWW\*J Low PASS FILTER Fig.2

INVENTOIQ ATTORNEY p 5, 1939. w. 5. PERCWAL 2,171,671

THERMIONIC VALVE CIRCUIT Filed May 17. 1937 2 Sheets-Sheet 2 H1 14 PASS DELAY FILTER SYSTEM DELAY SYSTEM ATTQQWE'Y Patented Sept. 5, 1939 I UNITED STATES PATENT OFFICE I 2,171,671 THERMIONIC VALVE CIRCUIT penny of Great Britain In Great Britain May 20, 1936 I I y Application May 17, 1937, Serial No. 143,105

6 Claims.

This invention relates to thermionic valve circuit arrangements and has particular reference to amplifying circuits used in connection with the transmission or reception of signals in wireless or television systems or in connection with low or audible frequency systems such as gramophone and talking film recording and reproducing systems.

Y All such systems are liable to interference which may be given the general term mush", and which is relatively more noticeable in the higher frequency ranges of the modulation than in the lower frequency ranges. Terms such as Schrott effect, Johnson noise and needle scratch in the case of gramophone record reproduction,

are well known as. describing these forms of interference and the object of the invention is to reduce such interference.

In order to reduce needle scratch in the case of gramophone record reproduction and mush in the case of wireless reception, proposals have been made to reduce the relative amplitudes of the higher modulation frequencies. The loss of such higher frequencies results in lack of proper balance in the reproduction and when loud passages of music or speech are being reproduced suppression of the higher frequencies is unnecessary because the amplitude of the interference is relatively small and the interference cannot be heard. In an automatic system for attenuating the higher frequencies in order to reduce interference, the attenuating effect has been made dependent upon the general level .of the signals. In such a system it will be understood that the i5 attenuating-effect operates regardless of the relative amplitudes with' the higher frequencies so that even those of sufficient amplitude to mask interference become attenuated.

According to the present invention a thermi- 40 onic valve circuit arrangement is designed toattenuate signals above a predetermined frequency and below a predetermined amplitude. The signals to be attenuated are separated by means of a filter which need not have a sharp 5 out off characteristic, and a voltage obtained by rectifying the separated signal components may be employed to vary the frequency characteristics of a circuit through which the main signal passes. If automatic gain control is employed 50 in the circuit, this ,may be used for correcting the change in carrier amplitude. In the case of gramophone record reproduction, because needle scratch is of relatively high frequency, it can be heard during the reproduction of a low fre- 55 quency note of a given amplitude, whereas a high frequency note of the same amplitude may completely mask the interfering needle scratch. In applying the'invention to an amplifier circuit arrangement'for gramophone record reproduc- 4 tion therefore. the higher frequencies are only 6 attenuated if their general level is below alevel at which interference becomes sumciently audible for it to be preferable to attenuate the interference even though this is unavoidably accompanied by the loss of the higher modulation frel0 quencies'of the signal.

In the case of a television system, the Schrott effect becomes more pronounced at uniformly illuminated portions of an image than at portions containing rapid graduations of light and shade. 1 .At uniformly illuminated portions of an image no high frequency is required in order that the image may be reproduced correctly and the presence of high frequencies on the control grid of a receiving cathode ray oscillograph tube allows 20 the objectionable interference to appear. By removing such high frequencies there is consequently no loss of picture detail but the objectionable interference is removed.

In order that the invention may be more clear- 25 ly understood and readily carried into effect, some circuit arrangements embodying the invention will now be described by way of example with reference to the accompanying'drawings in which:

Figs. 1, 2 and 3 are diagrams showing alternative forms of circuit. I

' Referring to Fig. l, signals to be amplified are applied to input terminals l and 2 across which two fixed'resistances 3 and 4 are connected. Anamplifier 5 followed by a high pass filter, 6 is connected across the input terminals l and 2 and a pair of diodes 1 and 8 connected in oppositlon and provided with condensers 9 and leak resistances I0 serve to rectify the output from the high pass filter 6 and to apply bias potentials to another pair of oppositely connected diodes H and I2. The diodes H and I2 are biassed by batteries 23 and 24 and with a condenser I 3 form a shunt path across an amplifier M which is connected across the resistance 4 and supplies output terminals I 5 and i6.

Under conditions of low amplitude high frequency input, diodes II and I! will be of low impedance due to the bias from batteries 23 and 24 and hence condenser l3 will become effective to attenuate high frequency signals fed to amplifier 14. As the amplitude of the higher frequencies increase, the rectified output from diodes I and 8 will increase and cancel the standing 2 bias due to batteries 23 and 2d. The impedance of diodes H and I 2 will therefore increase and the shunting effect of the condenser 13 is decreased and the high frequencies are passed to the amplifier l4 without attenuation together with other frequencies in the range being handled. In

' order to ensure that amplitude distortion shall not occur, the signal inputvoltage to the amplifier II should be small.

In the modified arrangement shown in Fig. 2

the signals from the input terminals I and 2 arepermit only amplitudes above a certain value to pass' through. The amplitude selector con sist's of a potential divider comprising a resistance 20 and two diodes 2| and 22 in parallel and connected in opposition. Batteries 23 and 24 are arranged to apply a bias to the diodes and thus control the point at which the amplitude filter becomes inoperative.

In a further modification shown in Fig. 3, the arrangement of Fig. 1 is combined with that of Fig. 2 .and in addition, delay networks :are in-. cluded. The lower frequencies are passed from the input terminals l and 2 to the output terminals l and I6 through a delay network25 and a. low pass filter 26. A high pass filter 21 allows'the high frequencies to be passed direct to an amplifier 28 feeding the diodes 29 and 30 'odes-H and I2 inFig. l.

which correspond with the diodes I and 8 in Fig. 1. to the diodes 3| and 32 corresponding with di- The high pass filter 21 is followed in the main channel by a delay network 33. Thus, any suppression of the shunting effect of the diodes 3| and 32 due to high amplitude high frequency voltages being rectifled by the diodes 29 and 30 can be made to occur just before the high frequencies causing that .suppression have passed through thede- "lay network, and the signals will thus be accurately reproduced. In thecase of a television image for example, consisting of a picture containing a single white strip down the centre, the whole of the reproduced picture will be'covere'd with more or less fine grain interference due to a normal level ofinterfering mus The circuit will operate to remove all undesirable high frequencies including mush except just before and just'after the two transients in each scanning line corresponding with the two edges of the white strip in tbo'ficture.

Since the muslf may vary under diflerent conditions, the condenser IS in thecase of Fig.

1 may be made variable in order to. adjust the frequency .at which attenuation commences.

23 and 2l may be adjustablein order to vary I the point at which the diodes H and I2- become non-conducting. It will be understood that the diagrammatic circuits have been shown by way These diodes supply rectified voltages and low pass filters and delay networks may be employed.

I claim: 1. An amplifier circuit arrangement in which in order to reduce the effect of interfering signals, means are provided for attenuating signals above a predetermined frequency and below a predetermined amplitude, said means including a high frequency circuit and a unilaterally conducting device connected in shunt across said high frequency circuit, means providing a bias potential for said unilaterally conducting de- .vilze, a filter which passes the signals at and above the frequency of the signals to be attenu-- input and output terminals, at least one frequency selective path connected across the system between said input and output terminals, said path being constructed 0 pass the frequencies of said range outside apredetermined portion of the frequency range, and said path including a pair of oppositely biased diodes connected in opposition to provide an amplitude limiting characteristic.

4. In-a wave transmission system adapted 'to transmit waves of a wide frequency range, wave input and output terminals, at least one frequency selective path connected across the system between said input and output terminals, said path being constructed to pass the frequenciesv of said range outside a predetermined portion of the frequency range, and said path including a pair of oppositely biased diodes con-.

nected in opposition to provide an amplitude limiting characteristic, said predetermined portion being the high frequency portion of the said range, and-means, responsive to the amplitude of said high frequency portion, for controlling the conductivity .of said diodes.

5. In combination with an amplifier network, a source of waves of a wide frequency range v .coupled to the network input, a path of low impedance to the higher frequencies of the range connected across the network, said path including a pair of oppositely connected space discharge devices, means oppositely biasing said devices, and means, responsive to a predetermined amplitude increase of said higher frequencies, for automatically increasing the'im pedances of said devices.

.6. In combination with an amplifier network, a source of waves of a wide frequency range coupled to-the, network input, a path of low impedan'ce to the higher frequencies of the range connected across the network, said path including a pair of oppositely connected space discharge devices, means oppositely biasing said devices, means, responsive to a predetermined amplitude increase of said higher frequencies, for

automatically increasing the impedances of said devices, and additional means for independently transmitting the lower frequencies of said range through a path in shunt with the network portion which includes said first path.

. WILLIAM SPENCER PERCIVAL; 

